1. Technical Field
The present invention relates to mixer circuits and more particularly to sub-harmonic mixers implemented in complementary metal oxide semiconductor (CMOS) technology.
2. Description of the Related Art
Mainstream complementary metal oxide semiconductor (CMOS) technology for millimeter wave applications is gaining attention primarily due to its low cost. However, as frequency of operation for these devices increases, for the circuits realized using this technology, it becomes challenging to minimize uncertainties arising due to incomplete device modeling and process variations at the increased frequencies.
A mixer is an important part of any super heterodyne radio architecture. The choice of mixer topology at millimeter wave frequencies depends heavily on the amount of local oscillator (LO) power available to drive the mixer. The power governs the key performance parameters such as conversion gain, noise figure and linearity.
At millimeter wave frequencies the available power from a CMOS device is low; hence, topologies of mixers desirable in a millimeter wave system should be limited to ones that can operate at lower LO power levels. It is also advantageous to have the LO source operating at a lower frequency (preferably a sub-harmonic of the required LO frequency to down convert the RF signal) because of ease of implementation, availability of better well-characterized passives and better phase noise and tuning range with less susceptibility to parasitics. Such a LO source could be followed by a multiplier to drive the mixer in the receive and transmit chain or it could be used to form an input to a sub-harmonic mixer (one that effectively uses a harmonic of the applied LO signal to perform the mixing).
Mixers using Schottky barrier diodes are sometimes preferred at millimeter wave frequencies because of their high cut-off frequencies in the THz range. A structure using an anti-parallel diode pair is commonly used to implement a sub-harmonic mixer. Fundamental mode mixers are also implemented using these diodes. Structures of this type have a serious drawback. They require large LO power. The conversion loss and noise figure degrade rapidly with reduction in LO power.
Among field effect transistor (FET) based mixers there are gate pumped and drain pumped mixers. In gate pumped mixers the radio frequency (RF) and LO signals are applied at the gate of a FET and the intermediate frequency (IF) signal is taken from the drain. The mixing occurs due to the non-linear variations of the FET transconductance (gm) as a function of gate source voltage (Vgs). The FET is biased in the saturation region with Vgs close to the threshold voltage of the FET (Vth).
In a drain pumped mixer, the LO signal is applied at the drain, the RF signal is applied at the gate and the IF signal is taken from the drain. The mixing action occurs due to the non-linear dependence of gm on the drain source voltage (Vds). In this case, the FET is biased at the transition between linear and saturation regions where maximum non-linearities are generated.
The drain pumped mixer configuration has an advantage over its gate driven counterpart because of the inherent port isolation between the RF and LO frequencies. Further improvement to the drain pumped mixer topology results in a dual-gate mixer topology wherein the RF and LO signals are applied to different gates thereby further improving RF-LO isolation. Also, the noise figure in a drain pumped mixer is found to be lower than that of the gate driven mixer (where LO and RF signals are applied to the same gate terminal) because of the low DC bias current of the FET also implying a low power implementation.
There have been implementations for CMOS frequency doublers and drain pumped mixers driven by a fundamental LO signal or twice the LO signal generated by such a frequency doubler. In Yang, et. al., “A 28 GHz sub-harmonic mixer using LO doubler in 0.18 μm CMOS technology,” IEEE RFIC Dig. Of Technical Papers, San Francisco, Calif., Jun. 12-14, 2006, pp: 239-242, a frequency doubler similar to one in Yang et. al., “Low power fully differential frequency doubler,” IEEE Electronics Letters, vol. 39, no. 19, pp: 1388-1389, September 2003, is AC coupled to a RF transistor at its drain to form a sub-harmonic mixer. However, these devices suffer from serious drawbacks, e.g.: (i) the drain of the RF transistor is floating and hence the transistor is not biased properly at DC, (ii) the circuit suffers from high conversion loss as well as noise figure due to its complete passive implementation, (iii) the device is not suitable for very high frequency applications because of the stacked push-push NMOS/PMOS implementation which results in more parasitics at the LO port.